The os trigonum is a small, extra bone that sits behind the ankle joint, present in roughly one out of every eleven people. Most who have one will never know it, because on its own the bone causes no symptoms at all. It becomes clinically relevant only when it gets pinched during forceful downward pointing of the foot, a condition that disproportionately affects ballet dancers, soccer players, and other athletes who spend a lot of time in that position. The gap between “having the bone” and “having a problem” is wide, and understanding where you fall in it makes all the difference in how to handle posterior ankle pain.
A Bone That Was Supposed to Fuse
During adolescence, a secondary center of bone growth appears at the back of the talus, which is the bone that sits between your shin and your foot. In most people, this growth center fuses with the talus by the mid-to-late teenage years, becoming part of a bony ridge called the posterior talar process. When it fails to fuse, it remains as a separate little bone connected to the talus by a strip of fibrous cartilage. That separate bone is the os trigonum, typically triangular or oval, usually no bigger than a marble.
A pediatric study tracking fusion patterns found that about 70% of adolescents whose os trigonum appeared on X-rays went on to achieve complete fusion by age 17, with girls fusing slightly earlier on average than boys.1Journal of Pediatric Orthopaedics. Incidence and Fusion of Os Trigonum in a Health Pediatric Population In other words, many people temporarily have an os trigonum during their growth years, but it simply becomes part of the talus before they finish growing. The ones who keep it into adulthood are a smaller subset.
How Common It Is
Reported prevalence numbers for os trigonum vary wildly depending on the imaging method used. A 2024 meta-analysis pooling data across many studies found an overall prevalence of about 9% of feet examined, with roughly a third of those cases being bilateral, meaning the bone showed up on both sides.2PubMed Central. Prevalence and clinical aspects of os trigonum: a meta-analysis That 9% figure represents a blend of studies using X-rays, CT scans, MRI, and cadaver dissection.
When researchers looked specifically with CT, which is far better at spotting small bones than a standard X-ray, the numbers jumped. One CT-based study of over 600 patients found os trigonum in about a third of them.3PubMed. Prevalence of Os Trigonum on CT Imaging That same study found the bone was present even in about 24% of ankles with no symptoms, confirming that having the bone is not the same as having a problem. The practical takeaway is that os trigonum is not rare at all, and its apparent rarity in older literature likely reflects the limitations of plain X-rays.
When It Starts Causing Pain
The os trigonum sits in a tight space behind the ankle. When you point your foot sharply downward, the back of the ankle compresses. If you have an os trigonum, the bone and its cartilage connection to the talus can get crushed between the talus above and the heel bone below. Do this once hard enough and you can injure the cartilage bridge. Do it repeatedly and you get chronic irritation, swelling, and pain at the back of the ankle. This is what clinicians call os trigonum syndrome, which is a specific form of posterior ankle impingement.
A study of 30 cases of posterior ankle impingement found that an os trigonum or a fracture of the posterior talar process was visible on imaging in about two-thirds of them, with the remainder involving an intact but prominent posterior process that simply got pinched.4PubMed Central. Posterior ankle impingement So the os trigonum is the most common structural reason for posterior impingement, but it is not the only one. People with a naturally long posterior talar process can develop the same syndrome without a separate bone.
Symptoms typically include a deep, aching pain at the back of the ankle that gets worse with activities requiring you to push off or point your toes. Swelling behind the ankle is common, and some people notice stiffness or a feeling of something catching in the joint.
Ballet Dancers and Other At-Risk Groups
Ballet gets the most attention in the medical literature because the en pointe position forces the ankle into maximal plantarflexion hundreds of times per rehearsal. Both the os trigonum itself and a prominent lateral talar process have been documented as common culprits behind posterior ankle impingement in dancers.5PubMed. Pathoanatomy of posterior ankle impingement in ballet dancers In one surgical series focused entirely on dancers, all twelve patients reported pain localized to the back of the ankle, with additional complaints ranging from weakness to swelling to nerve-related symptoms in the foot.6PubMed. Os trigonum impingement in dancers
But ballet dancers are not alone. Soccer players who strike the ball with the instep forcefully plantarflex the ankle on every kick. Swimmers who rely on a powerful flutter kick, downhill runners absorbing steep descents, and gymnasts landing from tumbling passes all load the back of the ankle in similar ways. Any sport or activity that combines repetitive toe-pointing with sudden force can push a quiet os trigonum into becoming a symptomatic one.
A cross-sectional study of over 1,100 young athletes found that os trigonum was more common in athletes whose growth plates had already closed than in those still growing.7PubMed. Developmental Patterns and Clinical Relevance of Os Trigonum, Os Subfibulare, and Os Subtibiale in Young Athletes This fits the developmental story: the ossicle only persists as a separate bone once the window for fusion has closed, so it tends to show up as a problem in late adolescence and early adulthood rather than in younger children.
The Flexor Hallucis Longus Connection
Running right alongside the os trigonum is the tendon of the flexor hallucis longus, the muscle responsible for curling your big toe downward. The tendon passes through a narrow groove between the two bumps of the posterior talus, and when an os trigonum is present, the anatomy of that groove changes. The extra bone can press against the tendon or its surrounding sheath, leading to inflammation called tenosynovitis.8PubMed Central. The Effect of Flexor Hallucis Longus Morphology on Os Trigonum Syndrome
This relationship is so consistent that os trigonum syndrome and flexor hallucis longus tenosynovitis are often discussed as overlapping conditions rather than separate diagnoses.9Clinics in Sports Medicine. Os Trigonum Syndrome and Flexor Hallucis Longus Tenosynovitis In ballet dancers especially, the combination is sometimes called “dancer’s tendinitis,” presenting as pain at the back of the ankle that radiates toward the inner side of the foot and worsens with relevé or en pointe positions.10PubMed. Stenosing tenosynovitis of the flexor hallucis longus tendon and posterior impingement upon the os trigonum in ballet dancers When posterior ankle pain is accompanied by triggering, catching, or pain with big-toe motion, the flexor hallucis longus should be high on the list of structures to evaluate.
How It Is Diagnosed
On physical exam, the hallmark finding is tenderness deep behind the ankle, between the Achilles tendon and the inner ankle bone. Pushing the foot into maximum plantarflexion reproduces or worsens the pain. These clinical findings are suggestive but not specific, because other conditions such as Achilles tendon problems or subtalar joint issues can produce similar complaints.
Standard lateral ankle X-rays can show the os trigonum as a small, well-rounded bone sitting behind the talus, though plain films miss smaller ones. MRI is more useful for diagnosing the syndrome rather than just the bone. On MRI, a symptomatic os trigonum typically shows bone marrow edema within the ossicle, swelling in the surrounding soft tissues, and sometimes erosion along the cartilage joint between the os trigonum and the talus.11Applied Radiology. Posterior ankle impingement syndrome due to os trigonum A study examining accessory bones and their associated pathology on MRI confirmed that bone marrow edema was the most frequently identified abnormality, with increased fluid in the tendon sheath being the most common tendon-related finding.12PubMed. The coexistence of os trigonum, accessory navicular bone and os peroneum and associated tendon and bone pathologies
When imaging and clinical findings are ambiguous, a diagnostic injection can settle the question. A small amount of local anesthetic is injected under fluoroscopic or ultrasound guidance directly into the space between the os trigonum and the talus.13PubMed Central. The diagnosis of the os trigonum syndrome with a fluoroscopically controlled injection of local anesthetic If the patient’s pain disappears within minutes, it strongly confirms that the os trigonum is the source. This approach also has some predictive value for surgical outcomes: patients who respond well to the injection tend to do well with excision. In some cases, the injection includes a corticosteroid along with the anesthetic, serving double duty as both a diagnostic and therapeutic intervention.14Insights into Imaging. Ultrasound Guided Injections in Patients Presenting with a Painful Os Trigonum or Prominent Posterior Talar Process
Conservative Treatment
Not everyone with a symptomatic os trigonum needs surgery. The first line of treatment is rest from the aggravating activity, anti-inflammatory medication, and sometimes a period of immobilization in a walking boot to calm the acute inflammation. Physical therapy focused on ankle mobility, calf and intrinsic foot strengthening, and activity modification can be enough for milder cases.
Corticosteroid injections, as described above, can provide weeks to months of relief and are worth trying before considering surgery. Some patients, particularly those whose symptoms are tied to a single intense episode rather than years of repetitive loading, respond well enough to conservative care that they never need the bone removed. The evidence on exactly what percentage of patients avoid surgery with conservative management alone is thin, but the general clinical approach is to give non-operative treatment at least three to six months before discussing excision.
Open Versus Endoscopic Surgery
When conservative treatment fails, the os trigonum is surgically removed. The bone is expendable and its removal does not compromise the structural integrity of the ankle. Two main approaches exist: traditional open surgery through an incision on the inner or outer side of the ankle, and minimally invasive endoscopic or arthroscopic surgery using small portals and a camera.
A randomized controlled trial with five years of follow-up compared the two approaches in athletes. Patients who underwent endoscopic excision returned to training in an average of about five weeks, compared to roughly ten weeks for the open surgery group. Time to return to their previous competitive level was about seven weeks endoscopically versus nearly twelve weeks with open surgery. The complication rate was also strikingly different: about 4% for the endoscopic group versus 23% for the open group.15PubMed. Endoscopic Versus Open Excision of Os Trigonum for the Treatment of Posterior Ankle Impingement Syndrome in an Athletic Population
A systematic review pooling multiple studies confirmed the trend, finding that the arthroscopic group returned to activity in an average of about ten weeks compared to thirteen weeks for the open group. Sural nerve injury, a complication that causes numbness or tingling along the outer edge of the foot, occurred in about 5% of open cases but fewer than 1% of arthroscopic ones.16The Journal of Foot and Ankle Surgery. Return to activity after excision of the symptomatic Os trigonum: A systematic review comparing arthroscopic versus open excision These advantages have made endoscopic techniques the preferred approach at most centers experienced with the procedure, though open excision remains a perfectly valid option in settings where the endoscopic equipment or expertise is not available.
It is worth noting that “endoscopic” and “arthroscopic” are sometimes used interchangeably in this literature, and there are actually subtle differences between posterior endoscopic approaches and standard arthroscopic approaches from different portal positions. One study comparing arthroscopic versus posterior endoscopic excision found no significant difference in return-to-sport time between the two minimally invasive methods, with both averaging around seven to eight weeks.17PubMed. Arthroscopic versus posterior endoscopic excision of a symptomatic os trigonum: a retrospective cohort study The bigger divide, in other words, is between open and minimally invasive surgery rather than between specific minimally invasive techniques.
Complications Worth Knowing About
Overall, os trigonum excision is considered a successful procedure, and most patients return to full activity. But it is not risk-free. The most talked-about complication is sural nerve injury with open approaches, as noted above, which can leave a patch of numbness on the outer foot that may or may not resolve. Endoscopic approaches carry a smaller but real risk of damage to other structures in the posterior ankle, including the tibial nerve and the flexor tendons.
A study specifically tracking complications after resection found a major complication rate of about 13% in a cohort of 30 feet treated with open excision. All four major complications involved persistent bone marrow swelling in the talus after the os trigonum was removed, and three of those four developed early arthritis in the subtalar joint beneath.18Foot & Ankle Orthopaedics. Complications After Resection of Os Trigonum or Posterior Talar Process That complication rate is higher than most surgical series report, and it came from a cohort that included both os trigonum excision and posterior talar process resection, which is a more aggressive procedure. Still, it is a reminder that removing a bone from the back of the ankle, even a small one, can alter the local mechanics enough to create new problems in a minority of patients.
Pediatric and Adolescent Patterns
The os trigonum occupies an unusual position in pediatric orthopedics because it straddles the line between a normal developmental variant and a potential source of pain. In one pediatric population study, about 18% of children and teens showed the bone on X-rays, but the majority went on to have it fuse naturally by age 17.19Journal of Pediatric Orthopaedics. Incidence and Fusion of Os Trigonum in a Health Pediatric Population Fusion was linked to skeletal maturity: when the os trigonum had fused, the patient had always reached at least an advanced stage of heel bone development.20PubMed Central. Posterior ankle impingement syndrome and os trigonum relationship in children
This creates a diagnostic puzzle for young athletes. A 13-year-old ballet student with posterior ankle pain and a visible ossicle behind the talus might have a symptomatic os trigonum, or might have a normal secondary ossification center that is going to fuse on its own in a couple of years. Distinguishing between the two often comes down to clinical symptoms, MRI findings of edema, and response to rest and conservative treatment. Jumping to surgery in a young teenager is generally avoided unless symptoms persist despite months of appropriate non-operative care. A small surgical series of children aged 11 to 17 who did undergo open excision reported complete pain relief and return to unrestricted activity within three months, suggesting that when surgery is clearly indicated in this age group, outcomes are good.21Orthopaedics & Traumatology: Surgery & Research. Surgical treatment of the symptomatic os trigonum in children
Differentiating Os Trigonum From a Fracture
One of the trickier clinical scenarios involves an acute ankle injury where a lateral X-ray reveals a small bone fragment behind the talus. Is it a pre-existing os trigonum that happened to show up on the injury film, or is it a fresh fracture of the posterior talar process, sometimes called a Shepherd’s fracture? The distinction matters because a fracture may need fixation or immobilization, while an incidental os trigonum needs nothing.
The key radiographic difference is the edges. An os trigonum has smooth, well-rounded cortical margins all the way around because it has been a separate bone for years. A fresh fracture fragment has irregular, jagged edges that look like a broken piece of something larger. On CT or MRI, the cartilage bridge of an os trigonum is visible as a distinct synchondrosis, while a fracture line cuts through previously solid bone. Comparison films of the opposite ankle can also help: if the same-looking bone is present on the uninjured side, it is almost certainly a longstanding os trigonum rather than a new fracture.
In practice, the distinction still trips up clinicians, especially in emergency settings where only a quick lateral X-ray is available. If you have been told you have an os trigonum fracture after an ankle injury and the recommendation feels uncertain, an MRI or CT can usually settle the question definitively.